Acceleration sensors have been hitherto used in various application fields. For example, when these acceleration sensors are applied to a vehicle field, they are used to detect acceleration under a travel state and perform various kinds of travel control such as ABS (Antilock Braking System), an air-bag system, etc. and body control, and to take safety measures, etc. for vehicles.
Semiconductor acceleration sensors which are relatively compact in size and that can be manufactured at a low cost have been mainly used as acceleration sensors mounted in vehicles. Particularly, there have been hitherto frequently used semiconductor acceleration sensors, each of which is designed so as to include a semiconductor sensor element for outputting an electrical signal in connection with acceleration and an amplifier for amplifying the electrical signal, the semiconductor sensor element and the amplifier being integrally mounted in a package.
Such a semiconductor acceleration sensor is subjected to various kinds of tests and calibration processing in a manufacturing process, and it is tested at the final stage of the manufacturing process whether it has predetermined electrical characteristics under acceleration-applied state. Specifically, a semiconductor acceleration sensor under the test is fixed on a vibrating table by a socket or the like, and the vibrating table is vibrated by a vibrating machine at a predetermined frequency to apply a constant acceleration to the semiconductor acceleration sensor for testing the various kinds of electrical characteristics (for example, see JP-A-10-232246, the contents of which are incorporated herein by reference).
Recently, high functionalization has been promoted for acceleration sensors, and acceleration sensors each of which is equipped with a low pass filter having a cut-off frequency of several hundred Hz (hereinafter referred to as “LPF”) have been mass-produced. Specifically, such a semiconductor acceleration sensor 100 as shown in FIG. 14 is known. This semiconductor acceleration sensor 100 is designed so that a predetermined frequency component is extracted from an electrical signal output from a sensor element 111 through LPF 112 and then amplified by an amplifier to achieve a sensor output signal. Since LPF 112 is equipped in the acceleration sensor as described above, it has been required that the mechanical frequency characteristic of the sensor element 111 and the electrical frequency characteristic of LPF 112 are totally guaranteed as a sensor assembly (the overall semiconductor acceleration sensor 100).
In order to guarantee the electrical frequency characteristic of LPF 112 (hereinafter referred to merely as “frequency characteristic” unless otherwise specified), it is required to measure the sensor output signal in the above-described testing work using a vibrating machine while varying the vibration frequency to each of plural frequencies (that is, the frequency of the electrical signal from the sensor element 111 is varied to plural frequencies). That is, for example, the vibration frequency is varied to 50 Hz, 100 Hz, 200 Hz, 300 Hz, and 400 Hz, and the gain at each frequency is measured as shown in FIG. 15. In this case, if each measurement value is within a specified range at each frequency, it could be judged that the frequency characteristic is normal. FIG. 15 shows a frequency characteristic when the cut-off frequency of LPF 112 is equal to 400 Hz.
However, there is a time delay from the time when the semiconductor acceleration sensor 100 is vibrated at a predetermined vibration frequency by the vibrating machine to the time when the acceleration is stabilized. Therefore, when the semiconductor acceleration sensor is tested while the vibration frequency is varied to each of the plural frequencies as described above, much time must be taken for the testing work.
Furthermore, at a high vibration frequency (for example, not less than 200 Hz), the fixing state of the semiconductor acceleration sensor on the vibrating table through the socket or the like becomes unstable. Particularly, in the case of the testing system disclosed in JP-A-10-232246, the vibrating table is mounted on the vibrating machine by a spring. Therefore, when the vibrating table is vibrated at a high frequency, the acceleration is not well transferred to the sensor assembly, and thus it is difficult to test the frequency characteristic accurately.
Therefore, under the present condition, the semiconductor acceleration sensor is vibrated at a low frequency (for example, not more than 100 Hz) at which the acceleration can be stably applied by the vibrating machine, and the attenuation amount, the phase characteristic, etc. under this condition are measured. In this case, with respect to high-frequency components such as the cut-off frequency of LPF 112, etc., the sensor assembly is not actually vibrated, but only LPF 112 is tested as a single unit through a wafer test in the manufacturing process. Therefore, there is a problem in that the frequency characteristic test of the sensor assembly itself cannot be accurately carried out.